How sizing systems work
Barleycorn system
US, UK, Australia, India
Each full size equals 1/3 inch (8.46 mm). Half sizes are 1/6 inch (4.23 mm). Different countries use different starting points, which is why US 9 does not equal UK 9.
Paris Point system
Europe, China
Each size equals 2/3 cm (6.67 mm). Sizes are unisex: EU 42 indicates 27.0 cm for men and women.
Mondopoint system
Japan, Korea, Ski Boots
Shoe size equals actual foot length in centimeters (Japan) or millimeters (Korea). The international standard is ISO 9407:2019.